1. Field of the Invention
This invention relates to digital line sensors, and, in particular, to digital energy line sensors which find utility in telecommunication systems. Accordingly, it is a general object of this invention to provide new and improved sensors of such character.
2. Description of the Prior Art
This invention is concerned with a communication system having an instrument at each end of a communications link. The instrument at each end is required to be advised when the other end wants it to respond. Each end, when it desires to initiate a conversation, notifies the other end that it should ring, buzz, turn itself on, or otherwise notify that the near end wants to talk. Thus, there are two goals: (1) letting the other end know that you wish to talk, and (2) when the other end wants to talk, letting you know.
In the past, in an ordinary telephone system, when the far end wanted to talk to you, a large ringing voltage was transmitted through the line, actuating and causing a clapper to move back and forth, hitting a bell. In the other direction, when you wished to talk to a central office, you picked up your handset, completing a circuit. Current begins to flow in a twisted pair loop and a dc current detector at the central office detects that some current has started to flow, initiating the system, providing a dial tone which indicates: "Yes, I have recognized your request to communicate with me, now dial".
Various U.S. patents, dealing with related technology, may be of interest.
U.S. Pat. No. 3,550,134 to Kurth et al, issued Dec. 22, 1970, discloses a double carrier bilateral transmission system which includes a transceiver at each end connected by a lossy transmission medium which introduces significant phase shift and attenuation to a modulated carrier passing therethrough. Each transceiver comprises a phase locked loop which transmits and receives modulated carriers having different carrier frequencies where the different frequencies are integral multiples of each other. One transceiver includes an auxiliary control circuit and an automatic gain control circuit to compensate for the phase shift and attenuation suffered by the modulated carrier in the transmission medium, respectively.
U.S. Pat. Nos. 3,546,703 and 3,550,131 to Kurth, issued Dec. 8 and 22, 1970, respectively, disclose frequency modulated analog systems which may be used for telephone transmission. The received analog system is converted to digital pulses and demodulated with a digital phase locked loop. The conversion is accomplished by means similar to conventional means for receiving digital signals, i.e., a level sensor (detector) followed by a pulse former (regenerator). The signal is then fed into a differentiator. Kurth detects changes in voltage on the transmission line which is proportional to line frequency which is proportional to the modulated analog signal desired. Disadvantageously, it is noted that frequency is measured in lieu of the presence of energy.
U.S. Pat. No. 3,341,815 to Axe, issued Sept. 12, 1967, discloses a receiving system with outputs to actuable means which may include a Schmitt trigger circuit. In a broad sense, Axe utilizes a detector (as above) followed by a regenerator (Schmitt trigger circuit). It is noted, however, that Axe does not suggest integration which is necessary to detect energy.
U.S. Pat. No. 4,056,694 to Brolin, issued Nov. 1, 1977, discloses a telephone ringing signal detector for detecting four party fully selective superimposed ringing signals on a telephone line. Detecting these signals at the central office end of a subscriber loop carrier system permits encoding the ringing signals for transmission on the carrier system and subsequent decoding and utilization at a remote carrier terminal to control the regeneration of selective ringing signals. The ringing detector includes separate 20 Hz alternating circuit signal detectors for each conductor of the telephone pair and further includes a separate polarity detector for each conductor. The polarity signals for the two conductors are gated by the outputs of the respective 20 Hz signals and combined in a single polarity indication applicable for either conductor. False signaling indications are thereby prevented in the presence of central office variations of amplitude, waveform and level of the dc signals, as well as variations in the phase of the ringing signals. Disadvantageously, Brolin does not suggest circuitry for use in a telephone to detect a central office request for service (ring), nor circuitry for the central office to detect a telephone request for service.
U.S. Pat. No. 4,048,448 to Canniff et al, issued Sept. 13, 1977, discloses a subscriber loop digital carrier system with a ringer encoder similar to that disclosed by Brolin, above. It also discloses a means for receiving an encoded ring signal and applying appropriate ringing voltages at the remote terminal end. Disadvantageously, Canniff et al is unduly power consuming.
U.S. Pat. No. 4,132,864 to Feng, issued Jan. 2, 1979, discloses the detection of a dc current, either alone or in the presence of a higher ac ringing current. Feng utilizes high frequency pulses to measure the duty cycle of loop current, employing digital detection circuitry. Further, Feng utilizes a high frequency master clock, and records the duration of a comparator output by summing master clock signals. Disadvantageously, a dc current as detected by Feng may no longer be present in digital telephones, and Feng's disclosed approach cannot be used in the telephone itself as a replacement for ringing voltages.
U.S. Pat. No. 3,359,372 to DeBurro, issued Dec. 19, 1967, relates to telephone signaling, and discloses a means for multiplexing two signal bits in a single digit space without creating a false framing pattern. Signaling detectors and regenerators are also disclosed. Disadvantageously, DeBurro does not suggest circuitry for use in a telephone to detect a central office request for service (ring), nor circuitry for use in the central office to detect a telephone request for service.
U.S. Pat. No. 3,336,444 to Piechocki, issued Aug. 15, 1967, discloses a telephone sensor system wherein a detector of binary signals trips a bi-stable trigger circuit (regenerator) providing a stream of pulses for both comparison purposes and for tripping a monostable trigger circuit. Disadvantageously, Piechocki does not suggest integration, and, though a Schmitt trigger is disclosed for converting sine waves into pulses, it does not have any direct bearing on detection.
In a digital telephone transmission system, in the ringing direction, large ac ringing voltages can no longer be sent, since they would tend to destroy the telephone circuitry. In the other direction, the telephone instrument that one is using may no longer be powered from the central office, so that when one goes off hook (turns on the instrument), it may no longer draw a current from the central office.
If a ringing voltage cannot be transmitted, or if a dc current is unavailable, how does one signal back and forth one's intent? An additional requirement for the system is that it be of low power so that, when the telephone handset is not in service, it consumes very little power. Preferably, it should be some kind of a sensor on the communications link because electrically conductive wire is being replaced with optical fiber; thus, no voltage and no current can be sent from one end of the link to the other.
What are the possible solutions? One solution, though complicated, would be to have a receiver continously operative that would recognize a special coded signal from the far end that it should do something (i.e., turn on a ringer, flash a light, send dial tone down, etc.). U.S. Pat. No. 3,336,444 to Piechocki describes a radio system which, when it receives the beginning of a bit stream, trips a trigger circuit which starts a shift register, compares a code stored in the shift register with the code received over the air, and at the end of the comparison, trips a ringer, a flashing light, or something else. Disadvantageously, all of the detector circuitry must be kept on, having the receiver continually distinguishing ones and zeros. Any noise spike would set off his circuit, initiating comparison of noise with its stored code, and then turning itself off because the noise would not be the internal code. Disadvantageously, it is not desired that noise initiate a call every time that some impulse noise comes down the line. It is not desired to have complex, sophisticated circuitry always being ready to turn on.